This invention relates to a cable structure of a submarine cable using a low-loss optical fiber as a transmission medium.
The optical fiber has the advantages of low loss, wide bandwidth and light weight and hence is regarded as a promising transmission medium which will succeed a coaxial submarine cable heretofore employed. A submarine cable is held as deep under the sea as 8000 m and exposed to a high pressure of about 800 kg/cm.sup.2 on the sea bottom. The optical fiber is made of low-loss silica glass or optical glass, and hence is brittle and easily broken under an external force, such as bending or tensile force; and when the optical fiber is immersed in seawater for a long time, a very small crack in the glass is likely to grow, resulting in breakage of the optical fiber. Therefore, the optical fiber is usually coated with nylon or like material so as to enhance its mechanical properties and prevent their degradation. In a case of this coated optical fiber, however, when it is given hydraulic pressure or the like, the optical fiber is slightly bent due to nonhomogeneity of the coating material, causing a marked increase in transmission loss.
In view of the above, it has already been proposed to house the optical fiber in a high pressure resisting pipe of a small diameter (refer to Japanese Pat. Disc. No. 99032/76). As the small-diametered, high pressure resisting pipe for housing the optical fiber, what is called a seamless pipe or welded pipe is considered; but, in a case of the seamless pipe, it is difficult to fabricate a long pipe while inserting thereinto the optical fiber, and it is also difficult to insert the optical fiber into the long pipe after it is produced. In a case of welded pipe, the temperature of a welding point raises, so that the optical fiber housed in the pipe may be sometimes damaged; therefore, some measure must be taken to protect the optical fiber from heating.